1. Field of the Invention
The invention relates to a non-linear optical device and notably to a device comprising a non-linear medium contained in a resonant cavity and coupled to a laser beam.
2. Description of the Prior
Many standard devices using non-linear optics (such as optical parametrical oscillators of frequency doublers) are constituted by two essential elements: firstly, the non-linear medium and, secondly, a resonant cavity. The latter is used, in the case of frequency doubling, to increase the conversion efficiency and, in the case of the optical parametrical oscillator (O.P.O.) to produce laser-type beams of adjustable frequency. In the simplest case, the cavity is constituted by two concave mirrors, the faces of which are treated so as to have a high reflection coefficient for the useful wavelength range. The treatment of these mirrors consists of stacks of dielectric layers. They generally constitute one of the following reasons:
It is difficult to manufacture highly reflective mirrors on a wide spectral band (for O.P.O. use). The complexity of the treatment necessary makes the mirror fragile and lower its damage threshold, thus limiting the power that can be admitted into the cavity and the focusing of the beams;
It is difficult to achieve very precise control over the reflection coefficients of the mirrors during manufacture. Furthermore, if it is sought to optimize the overvoltage coefficient of the cavity for the application envisaged, it is necessary to have a series of different transmission mirrors available, thus increasing costs;
The mirrors are made on substrates which must be as transparent as possible in the range of useful wavelengths. Apart from the manufacturing difficulties for certain of these substrates, they often show residual absorption due to impurities which are present in the state of traces (O--H bonds for example) and make the mirrors unusable at the wavelengths that correspond to these absorptions.
It is difficult to make mirrors with good reflection coefficients in the short wavelengths (U.V.) range whereas there are transparent non-linear crystals available in this range;
When the crystal is placed in the cavity, it is necessary for both its faces to be provided with anti-reflection treatment in order to limit the accidental losses which are responsible, in the case of the O.P.O., for a rise in the oscillation threshold. The anti-reflection treatment generally has lower damage thresholds than that of the crystal itself and therefore constitutes a limit to its optimal use.
These drawbacks mean that it is worthwhile to seek a resonant cavity configuration in which the mirror and the multi-dielectric treatment are eliminated.
The invention provides an approach by which these problems are resolved.